Lubricants are generally employed in metalworking operations. Such operations include rolling, forging, blanking, bending, stamping, drawing, cutting, punching, spinning, extruding, coining, hobbing, swaging, and the like. The present invention concerns lubricants for such type of metalworking operations, and in particular such operations as employed in automotive applications. In the automotive field, the term "stamping" is used as a broad term to cover all pressworking operations on sheet metal, which operations may be further categorized as cutting, drawing, or coining. Automotive stamped parts may be produced by one or a combination of these three fundamental operations.
Metalworking lubricants facilitate these operations generally by reducing friction between the metal being worked and the element employed for that process, and thus reducing the power required for a given operation, reducing the wear of the surfaces of the work elements that operate on the metals, and preventing sticking between the metal being worked and the work elements operating thereon or between metal pieces during storage, handling or operations, and in addition often provide corrosion protection to the metal being processed. In automotive applications prevention of sticking between metal pieces and between such pieces and the work elements is of extreme importance.
In some metalworking processes, including automotive applications, coils or rolls of steel, in particular cold rolled or galvanized steel sheets, are cut into pieces, called blanks, which are stamped or drawn to produce the desired parts. Such parts formed by stamping or drawing, as these terms are generally used, include fenders, hoods, deck lids, quarter panels, oil pans, fuel tanks, floor panels, inner and outer door panels, and the like. Prior to the use of lubricants known as prelubes, the normal procedure was to apply an oil at the steel mill to such coils or rolls as a rust preventative prior to shipping to the processing site, such as a stamping plant. Between the steps of cutting the sheets into blanks and stamping or drawing, such rust preventive oil would then be removed by washing and a drawing lubricant applied to the metal and at times the work element immediately before stamping or drawing. Such drawing lubricant is used to reduce friction and facilitate the metalworking operation.
In recent times the use of separate rust preventive oils and drawing lubricants has been in some instances replaced by the use of a single composition known as a PG,4 prelube. Prelubes are generally applied at the steel mill during temper rolling or inspection, as would be a rust preventive oil, prior to shipping and are not intentionally removed from the metal until after the blanks are formed and the parts formed. Thus the use of such prelubes eliminates the steps of removing the oil and applying a drawing lubricant before further working.
Prelubes thus must function as both a rust preventative and drawing lubricant. In many instances, and particularly for automotive applications, a prelube must be removable with alkaline cleaners, non-staining to the metal, and compatible with other chemicals utilized in producing the products in question.
In more detail, the advantages obtained by the efficiency of using a prelube would be diminished or nullified if unusual methods were necessary to remove the lubricant from the final product. In the automotive field, alkaline cleaners are the normal compositions employed for cleaning. Some substances with lubricating properties, for instance wax films, cannot be easily removed with alkaline cleaners and thus their use entails a serious detrimental effect on the efficiency of overall operations.
As to metal staining, there are at times instances where steel coils are stored for long periods before use. Some substances may oxidize to an extent during storage and the oxidation products adversely effect the metal, for instance by the oxidation of oils to fatty acids which stain steel sheets, particularly mild steel sheets. Hence industries in which storage periods are a potential require prelubes or other substances in contact with the metal during storage that are substantially nonstaining.
As to compatibility with other chemicals, parts are sometimes formed with severe bends which may entrap some of the lubricant used in the metalworking operation. Thus although the lubricant may be removed after working from all exposed surfaces, the entrapped portion remains and may be vaporized or otherwise released under subsequent processing conditions. The potential for releasing of entrapped lubricant thus requires compatibility between the lubricant and cathodic primers and automotive adhesives. The cathodic primers are used in electrostatic coating operations. Adhesives are used to bond parts where welding or other methods are unsuitable. Although some parts being formed in a typical stamping plant will not be painted nor come into contact with adhesives, and thus the use of noncompatible lubricants thereon would pose minimal risks, efficiency in the overall operations makes it highly desirable to utilize the same lubricant or prelube throughout the plant.
The prelubes now used commercially in the automotive industry are hydrocarbon based compositions containing sulfurized or waxy components, liquid at ambient room temperature. These compositions tend to drain off the metal surfaces, creating maintenance problems, and further tend to be or become unevenly distributed on the metal surfaces due to capillary forces. The properties of rust prevention and drawing assistance both depend in significant part on uniformity of lubricant film. Such tendency to puddle on the metal surface diminishes a lubricant's potential in providing protection from rust and in facilitating the stamping or drawing operations. Thus at least the automotive industry desires a prelube that provides lubricant film uniformity and film strength undiminished during shipping and storage periods. Further, film strength is a particularly significant property for facilitating drawing operations; a lubricant having high film strength will permit more severe draws to be made.
A lubricant that is effective for each of the purposes for which it is employed at use levels less than those otherwise encountered is desirable, contributing to the cost efficiency of any given operation.
It is an object of the present invention to provide a metalworking lubricant, and more particularly a lubricant that may be used as a prelube, that provides the foregoing desirable characteristics and permits the attainment of the foregoing advantages in the metalworking field, and in particular in the automotive industry. It is a further object of the present invention to provide a method of lubricating metal, particularly colled rolled and galvanized steel sheets, particularly prior to stamping and drawing operations, that provides the foregoing desired advantages. These and other objects of the invention are described below.